Gruntfile

This makes building the chrome extension very easy. You just have to drop in your private key `toggl-button.pem` and run `grunt` (actually `grunt --force` because jslint fails for now) which will run `grunt jslint`,  `grunt crx` and `grunt compress`. With this change people can easily modify the code and build their own, automatically updated, private extension which can be used with private gitlab installations for example.

For more info see https://github.com/oncletom/grunt-crx